Blue Jellyfish numbers up

The thousands of dead Blue Jellyfish (Catostylus mosaicus) on Port Phillip Bay beaches in recent months prompts the obvious questions: why are there suddenly so many; and why did they die?

Is it a natural cycle, or was it something they ate? Blue Jellies are said to feed on micro-organisms, small fishes, and organic particles floating in the water column.

Blue Jellyfish - what's going on there?

Until a couple of years ago, I’d found Blue Jellies in the Bay through spring and summer, but absent in winter. Their occurrence coincided with greater abundance of microscopic algae that they feed on, which bloom due to warmer conditions, increased daily hours of sunlight, and increased nutrient levels flushed into the Bay by spring rains and summer thunderstorms.

Although the year round presence of the Blue Jellies has occurred since the Channel Deepening Project, and dredging does release nutrients into the water column, it is impossible to tell if this is the main cause of the Blue Jelly explosion. Overfishing of their predator species in eastern Australian waters may be a contributing factor.

But it is interesting to note that the biomass of Blue Jellyfish was greater than the combined total of all other species recorded by the Baywide Anchovy Sub-program in both June 2008 and June 2009; and the greatest concentrations of Blue Jellies were in the central area of the Bay, a few kilometres south of the Northern Dredge Material Ground.